Friction materials have been used to brake cars, railroad vehicles and other various industrial machines. The current mainstreams of the friction materials are non-asbestos disc brake pads, and, as disclosed in Japanese Patent Application Examined Publication No. 59-4462 (1984) and Japanese Patent Application Unexamined Publication No. 6-184525 (1994), contain as reinforcing fibers combinations of metal fibers, such as steel fibers, brass fibers and copper fibers, organic fibers, such as acrylic fibers, aramid fibers and phenolic fibers, and inorganic fibers, such as rock wool, potassium titanate fibers, alumina-silica fibers and carbon fibers.
In view of heat resistance and strength, phenolic resins have generally been used as binders in the friction materials, but, under such severe conditions as frictionally sliding faces are heated to 300° C. or higher, apt to cause the fading phenomenon that the friction coefficient is reduced considerably because the binders are thermally decomposed into liquid decomposed matters and lubricate the sliding faces.
Decreasing the content of organic materials and controlling the porosity of friction materials have been the conventional means for preventing the fading phenomenon, but hardly solved the problem of the reduction of friction coefficient at elevated temperatures due to the fading phenomenon.
Further, the thin sliding film formed on frictional sliding faces and composed mainly of organic matters rapidly increases the friction coefficient on braking in a cold environment, causing the problematic, high frequency noise (grabbing chatter).
To solve these problems, abrasive materials of various hardness and particle diameters have been blended to grind the film on the frictional sliding faces and controlling the thickness, but could not reduce the thickness uniformly and failed to reduce the grabbing chatter both under normal conditions and after allowing to cool. For example, In Japanese Patent Application Unexamined Publication No. 2000-234086 disclosed is a friction material comprising a fiber component, a binder and friction adjusting components, wherein calcium oxide and calcium hydroxide are used as friction adjusting components in combination with other friction adjusting components, such as activated alumina. However, even the friction material cannot prevent sufficiently the reduction of friction coefficient and the occurrence of the fading phenomenon. Additionally, as matters now stand, it can hardly reduce the grabbing chatter after allowing to cool.